Thromboembolic diseases remain the leading cause of death in developed countries despite the availability of anticoagulants such as warfarin (COUMADIN®), heparin, low molecular weight heparins (LMWH), synthetic pentasaccharide factor Xa inhibitors, direct thrombin inhibitors such as Bivalirudin, and antiplatelet agents such as integrin αIIbβ3 inhibitors, aspirin, clopidogrel (PLAVIX®), and vorapaxar (Zontivity®). Additionally, current anti-platelet therapies have limitations including increased risk of bleeding as well as partial efficacy (relative cardiovascular risk reduction in the 20 to 30% range). Thus, there is an unmet medical need for safe and efficacious oral or parenteral antithrombotics for the prevention and treatment of a wide range of thromboembolic disorders.
Thrombin is a protease at the center of coagulation. In addition to the activation of coagulation factors and fibrinogen, thrombin regulates cellular activities through stimulation of the G-protein coupled protease activated receptors (PARs). These receptors are activated by cleavage by thrombin, and in a unique mechanism, the new amino terminus is the activating “tethered ligand.” This causes irreversible activation of the receptors. Platelets express two PARs, PAR-1 and PAR4. PAR-1 is ubiquitously expressed and PAR1 signaling underlies various conditions including coagulation, inflammation, nociception, healing and cancer metastasis. Conversely PAR4 expression is constrained, mainly to platelets and expression in certain brain areas and vascular beds after stress.
PAR1 is the “high affinity” thrombin receptor requiring little thrombin for activation whereas PAR4 is the low affinity thrombin receptor and requires significantly higher amounts of thrombin for activation. Due to this difference in affinity, PAR1 and PAR4 are engaged in a progressive manner, with PAR1 activated at low thrombin concentrations and PAR4 activated at higher thrombin concentrations. Because of the delay in activation we hypothesize that PAR4 antagonism might not affect hemostasis as potently and thus may be a better therapeutic target than PAR1.
Inhibitors of PAR1 have been investigated extensively, and several compounds, including vorapaxar and atopaxar have advanced into late stage clinical trials. Recently, in the TRACER phase III trial among non-ST-segment elevation acute coronary syndromes (ACS) patients, vorapaxar did not significantly reduce the primary composite endpoint, and in fact was halted early due to a significant increase in the risk of major bleeding, including intracranial hemorrhage (Tricoci, P. et al, N. Eng. J Med., 366(1):20-33 (2012). However, among non-ST-segment ACS patients undergoing CABG specifically, vorapaxar was associated with a significant reduction in ischemic events and no significant increase in major CABG-related bleeding (Whellan D J et al, J Am Coll Cardiol., 63(11): 1048-57(2014). The TRA 2P-TIMI 50 trial demonstrated that in patients with myocardial infarction, vorapaxar reduced the risk of cardiovascular death or ischemic events with a significant increase in moderate to severe bleeding when added to the standard anti-platelet therapy (Scirica B M et al, Lancet., 380(9850): 1317-24 (2012). Similar results were collected among patients with peripheral artery disease demonstrating significant beneficial effects on limb ischemia and peripheral revascularization with increased risk of bleeding (Bonaca M P et al, Circulation., 127(14): 1522-9 (2013). However, among patients with prior ischemic stroke adding vorapaxar to the standard of care increased the risk of intracranial hemorrhage without improvement in major vascular events (Morrow D A et al, Stroke 44(3):691-8 (2013). Although the PAR1 antagonist Vorapaxar (Zontivity™) was approved by the FDA as the first in class protease activated receptor antagonist, its potential application is severely limited by the bleeding side effects and increased risk of hemorrhagic stroke.
Interestingly, PAR4−/− mice are protected from thrombosis and cerebral ischemia-reperfusion injury and have prolonged tail bleeding times but no bleeding disorder. Since PAR4, is the low affinity thrombin receptor, Previous worksuggests that PAR4 would not be engaged until later stages of hemostasis, and possibly thrombosis. Thus PAR4 is an attractive target for a safer anti-platelet therapy for the treatment or prevention of thrombosis and cerebrovascular injury because inhibition of PAR4 preserves PAR1 as well as thromboxane, and purinergic receptor signaling which are major mediators of platelet activation. Therefore hemostasis is likely to be left intact and potential bleeding side effects could be ameleiorated or eliminated entirely.
There are several early reports of preclinical studies of PAR4 inhibitors. Lee, F-Y. et al., “Synthesis of 1-Benzyl-3-(5′-hydroxymethyl-2′-furyl)indazole Analogues as Novel Antiplatelet Agents”, J. Med. Chem., 44(22):3746-3749 (2001) discloses in the abstract that the compound:
“was found to be a selective and potent inhibitor or protease-activated receptor type 4 (PAR4)-dependent platelet activation.”
YD-3 was also referenced in Wu, C-C. et al, “Selective Inhibition of Protease-activated Receptor 4-dependent Platelet Activation by YD-3”, Thromb. Haemost., 87: 1026-1033 (2002). Also, see Chen, H. S. et al, “Synthesis and platelet activity”, J. Bioorg. Med. Chem., 16: 1262-1278 (2008).
EP1166785 A1, EP0667345, and WO 2013/163279, all incorporated herein by reference, disclose various compounds which are useful as inhibitors of platelet aggregation.